The present invention relates to mass spectrometry apparatuses and methods for obtaining data which identify the mass to charge ratio of various parent ions in a sample as well as mass to charge ratio of daughter ions produced by fragmentation of the parent ions in the sample, such as to determine structural information about the parent ions, and to derive other information about relationships between the parent ions and daughter ions. More particularly, this invention relates to mass spectrometry systems which include tandem mass analyzers separated by an ion fragmentation cell to obtain multi-dimensional data about the parent ions and daughter ions of the sample.
In simple mass spectrometers (MS), sample ions are formed in an ion source, such as by Electron Impact (EI), or by Atmosphere Pressure Ionization (API). The ions then pass through a mass analyzer, such as a quadrupole or time of flight device (TOF), for detection. The detected ions can be molecular ions (parent ions), fragment ions (daughter ions) of the molecular ions, or fragment ions of other daughter ions.
Quadrupole mass analyzers and magnetic sector mass analyzers, are mass filter type mass analyzers that allow only ions with specific mass/charge ratios (m/z) to pass through. Other ions are discarded during the scan. These type of mass analyzer is not non-destructive. This type of mass analyzer is thus not particularly effective for a full mass scan (also called full spectrum scan) where multiple ions of different m/z in a sample are to be detected and/or measured. Ion trap mass analyzers can trap ions and than analyze them sequentially based on the Fourier Transform Ion Cyclotron Resonance (FT-ICR) m/z. Mass analyzers can obtain similar full spectrum data, but in a different fashion by first measuring all of the ions and then performing a fourier transform analysis to measure the different ions in the sample. Therefore, the duty cycle and effectiveness of these types of non-destructive mass analyzers for full mass scans is higher than for mass filter type instruments. Time of flight mass analyzers sort ions based on flight time from an accelerator region to a detector spaced from the accelerator region. TOF mass analyzers can detect all ions, no matter what their mass to charge ratios are, and so they have very good sensitivity for a full mass scan spectrum.
Ion fragmentation mass spectrometers have been developed, characterized by having two or multiple sequential stages of mass analysis and an intermediate fragmentation region where parent ions from the first stage are fragmented into daughter ions for the second stage. Hence, these are generally termed xe2x80x9ctandemxe2x80x9d or xe2x80x9cMS/MSxe2x80x9d instruments. In such tandem mass spectrometers, sample ions are produced in an ion source, and the first stage of mass analysis analyzes selected parent ions of particular mass or m/z with a mass filter type mass analyzer. Then, some of the selected parent ions are fragmented or otherwise caused to dissociate, such as by metastable decomposition, collision induced dissociation (CID), or collisionally activated dissociation (CAD), to produce the daughter ions. Finally, the second stage of mass analysis sorts the daughter ions according to mass or m/z.
There are two styles of instruments in terms of xe2x80x9ctandemxe2x80x9d mass spectrometers, xe2x80x9ctandem in spacexe2x80x9d and xe2x80x9ctandem in time.xe2x80x9d Tandem in space mass spectrometers, such as triple quadrupoles and quadrupole-time of flight (Q-TOF) devices, have two mass analyzers, one for parent ion selection and one for daughter ion detection and/or measurement. Two mass analyzers are separated by a fragmentation device. Tandem in time instruments, on the other hand, have one mass analyzer that analyses both parent ions and daughter ions, but sequentially in time. Ion trap and FT-ICR are two most common mass spectrometers that have tandem in time MS/MS. The parent ions first are selected in the analyzer cell then fragmented. Often fragmentation takes place inside the analyzer. Then the daughter ions are analyzed in the same cell. Alternatively, it is known to analyze the daughter ions in a downstream analyzer, such as a TOF analyzer.
Several MS/MS scan types are used based on the relationship between the parent ions and the daughter ions. xe2x80x9cDaughter scanxe2x80x9d is a method that involves a full scan of daughter ions while the parent ion from which the daughter ions originate is pre-selected and fixed. This method is useful if an analyst knows the molecular weight of the parent ion and wants to know structural information about the parent ion. For instance, two distinct parent ions of similar molecular weight, but different structure can be differentiated by what daughter ions they typically fragment into. The data dependent daughter scan is often used when combined with liquid chromatographs (LC-MS/MS). The mass spectrometer automatically selects a parent ion peak based on previous scans and the peak intensity, charge state and other considerations. The mass analyzer then makes a full scan of the daughter ions resulting from fragmentation of the parent ion of interest.
xe2x80x9cParent ion scan,xe2x80x9d also known as xe2x80x9cprecursor scan,xe2x80x9d is a method that has a fixed daughter ion selection for the second analysis stage, while using the first stage to scan all of the pre-fragmentation parent ions in the sample. Only those molecules/compounds in the sample are detected which produce a specific daughter ion when fragmented. If both parent ion selection and daughter ion selection are fixed, an analyst will get selected reaction monitoring (SRM). SRM has the best selectivity, and good signal to noise ratio for quantitation.
xe2x80x9cNeutral loss scanxe2x80x9d is a method that shows all parent ions that lose a particular mass during fragmentation. The second stage mass analyzer scans the ions together with the first stage mass analyzer but with a certain offset. Neutral loss scans are used for screening experiments where a group of compounds all give the same loss.
Magnetic and electrostatic sector (together referred to as xe2x80x9csectorxe2x80x9d) mass analyzers have relatively slow scan speed, so sector based MS/MS instruments including sector-sector, sector-quadrupole and sector-TOF are normally good for daughter scans which don""t need high speed scanning of parent ions in the first stage. Tandem in time instruments select the parent ion first, then fragment and scan the daughter ions later. Normally this type of instrument can only perform full mass scan of the daughter ions.
Time of flight mass analyzers are known to have a number of advantages, including fast scanning rate, higher sensitivity, relatively high resolution and good mass accuracy. Q-TOF is a MS/MS instrument that combines quadrupole and TOF analyzers. It gives very good mass accuracy and sensitivity on full mass daughter scans but only filters a chosen parent ion with other parent ions being lost.
Triple quadrupole mass spectrometers can do all of the above scans. However, since both the first and second stages of mass analysis are of the mass filter type, triple quadrupole systems are generally less effective than ion trap for full scan MS/MS, and less accurate and sensitive than Q-TOF.
To solve modern analytical problems an analyst often needs to use more than one MS/MS scan method. For LC-MS/MS the parent ions duration time is limited because additional peaks elute from the LC device in a specified time period. Normally there is not enough time to do different types of scans in a single LC run. It is also not unusual that several parent ions co-elute at the same time. In many cases, data dependent scans do not have enough time to fully analyze all parent ions. A combined sector and TOF mass spectrometer is described in Enke at al U.S. Pat. No. 4,472,631. In Enke""s method, a collision cell is placed before a magnetic sector. A pulsed ion source is also used, so that the flight time of the ion can be measured. The time resolution is used for parent ion information while a spatial resolution from a sector is used to give daughter ion information. By using a digital computer, a partial two dimensional spectrum of the selected parent ion and daughter ions can be reconstructed.
In Enke""s invention, two spatial scan methods are described. One uses a fixed slit before the ion detector. Different daughter ion spectrums can be obtained by scanning magnetic field strength on the sector. For this method, only daughter ions with a particular m/z can be detected at a time. Daughter ions with a m/z other than this particular range of m/z will be thrown away. Less than 1% of all possible useful information can be obtained by the Enke device. This device is thus not effective to obtain highly sensitive full scan daughter ion spectrums.
A design using a multi-channel spatial array detector is also described by Enke. With this design, magnetic field strength within the magnetic sector is not scanned during operation. Rather, a micro-channel array, positioned at the focal plane of the magnetic sector, simultaneously detects and individually resolves ion currents from a plurality of ion paths by use of individual micro-channels. The individual outputs of the micro-channel array are connected through amplifiers to individual time array detectors, connected to a digital computer. This method provides much better detection efficiency with a high duty cycle, but the spatial resolution is limited by the number of detector arrays and the size of the instrument. For a high resolution measurement, thousands of detector elements and associated electronics would be needed.
Parent ions are first separated by a relatively slow, non-destructive scan device, for example, an ion trap. These parent ions are collected within the ion trap and then selectively released into a fragmentation device, such as a collision cell external to the first analyzer. Parent ion information is determined based on the time that individual parent ions are released from the ion trap or other first mass analyzer. The fragmentation devices sequentially fragment the parent ions into daughter ions. Than each daughter ion is analyzed by a fast scan analyzer, for example, a time of flight (TOF) mass analyzer.
In TOF scan, all ions from the same scan are originally from parent ions having the same mass/charge ratio (m/z). In a certain range, all ions will be fragmented and scanned by TOF scans. A complete two-dimensional MS/MS map can be obtained after a single ion trap scan. A full scan MS spectrum can also be reconstructed by plotting total ion counts for each TOF scan.
Different MS/MS scans such as daughter scan, parent scan, neutral loss scan and selected reaction monitoring are all subsets of this complete 2-D MS/MS map.
During the MS/MS scan, unlike ion filter type instruments, no unnecessary ion loss occurs. A multi-pole ion guide with an electric ion gate prior to the ion trap can also act as an ion reservoir during the scan. Therefore, a theoretical 100% efficiency can be achieved.
Accordingly, a primary object of the present invention is to provide apparatuses and methods for more rapidly, more completely, more flexibly and more efficiently obtaining data of the type obtained by tandem mass spectrometry (MS/MS).
Another object of the present invention is to provide an apparatus and method for rapidly obtaining ion mass data with high sensitivity and a large dynamic range.
Another object of the present invention is to provide a single mass spectrometry instrument that has good versatility and can perform in multiple scan modes.
Another object of the present invention is to provide a method and apparatus for obtaining MS/MS type two dimensional data about parent ions and daughter ions sufficiently rapidly to facilitate combination with a chromatographic apparatus, such that complete multidimensional data can be obtained in real time, during the relatively short duration of a single chromatographic peak.
Another object of the present invention is to provide a method and apparatus that uses a non-destructive mass analyzer for both first and second stage analysis for obtaining complete spectrum MS/MS type data.
Other further objects of the present invention will become apparent from a careful reading of the included drawing figures, the claims and detailed description of the invention.